A curable composition containing a 2-cyanoacrylic acid ester quickly establishes a strong bonding among various types of materials attributed to the unique anion polymerization characteristics exhibited by its principal component, 2-cyanoacrylic acid ester, which initiates polymerization under the presence of weak anion provided by slight moisture or the like attached to a surface of an adherend. Accordingly, it is used as the so-called instantaneous adhesive in a wide range of fields including industrial, medical, and household applications. Because the cured product thereof generally has no flexibility and is hard and brittle, there is a problem of impairing flexibility of adherends in case of bonding flexible adherends. Furthermore, the curable composition exhibits excellent shear adhesion strength; however, it is problematic in that peel adhesion strength and impact adhesion strength are low, and particularly cold/hot cycle resistance is inferior, particularly when it is applied between different types of adherends.
To overcome these problems, modification methods have been proposed heretofore, which include adding various types of plasticizers, elastomers and poorly-soluble rubber particulates (see, for instance, Patent Documents 1, 2 and 3).
Also proposed are an adhesion method that improves adhesiveness and a two-part curable composition. For example, Patent Document 4 discloses an adhesion method using two types of adhesives, i.e., (A) 2-cyanoacrylic acid ester and (B) an adhesive that is moisture-curable at ordinary temperature and is composed mainly of a polyisocyanate compound or an organic silicone compound having a hydrolyzable sily group, whereby temporary fixation is quickly achieved by the 2-cyanoacrylic acid ester, and sufficient peel adhesion strength and impact adhesion strength are exhibited after certain lapse of time. Further, Patent Document 5 discloses a two-part curable composition comprising a first reagent containing a 2-cyanoacrylic acid ester and a cation polymerization catalyst and a second reagent containing a cation curable component. Patent Document 6 discloses a two-part curable composition comprising a first reagent containing a 2-cyanoacrylic acid ester and a peroxide and a second reagent containing a radical-curable monomer and a transition metal.
On the other hand, also known in the art is a method for improving water-resistance of the curable composition by blending a monosilane compound or a bissilane compound in the 2-cyanoacrylic acid ester (e.g., Patent Document 7).